We studied anti-interferon (IFN) antibody in serial serum samples, obtained from 34 patients with chronic hepatitis C treated with IFN (HLBI, recombinant IFN-alpha 2a or-2b, HuIFN-beta). Anti-IFN antibody was measured by Western blotting, Anti-IFN antibody was detected in nine (8.1%) of the 111 samples. Two patients treated with recombinant IFN-alpha 2a developed neutralizing anti-IFN antibody, with the development of clinical resistance to IFN therapy, while none of 32 patients treated with either HLBI, recombinant IFN-alpha 2b or HuIFN-beta developed anti-IFN antibody. Anti-IFN antibody was developed in one patients one month after IFN. It is believed that Western blotting, used in this study, is a useful method to evaluate the incidence and clinical significance of anti-IFN antibody.